Millions of Americans visited new online health insurance exchanges as enrollment opened on Tuesday, suggesting a broad national appetite for the affordable coverage that President Obama has promised with his health care law. But many people quickly encountered technological problems that prevented them from getting rates, comparing health plans or signing up.

Federal and state officials said that while they knew there was pent-up demand for health coverage, the number of visits to their exchanges was greater than anticipated. Federal officials said more than 2.8 million people had visited HealthCare.gov, the federally run exchange that serves residents of more than 30 states, though the figure would include those who received error messages. State-run exchanges also reported higher-than-expected use, including several million visits to New York’s Web site.

The demand “exceeds anything that we had expected,” President Obama said at the White House on Tuesday afternoon.

But it remained unclear whether the array of problems — many people received messages saying the system was down, and others were unable to create accounts to buy insurance — stemmed more from heavy traffic or from flaws in design. Federal and state officials had promised for months that the exchanges would be ready for heavy use by Oct. 1, and had run numerous tests to ensure that the complex systems would work properly from the start.

The problems came on a landmark day for the health care law, Mr. Obama’s signature legislative achievement and the central issue in the Congressional battle over federal spending that has led to a shutdown of the government. A trouble-plagued start, accompanied by the complaints of frustrated consumers, may undermine political support for the law and discourage people from signing up.

Across the country, people trying to use the Internet exchanges expressed mixtures of hope and frustration.

At the Jessie Trice Community Health Center in Miami, out of about 70 people who arrived seeking help with the new program before noon, no one was able to get access to the marketplace. All were given appointments to come back another week.

At the Lower Light Christian Health Center, a clinic just west of downtown Columbus, Ohio, Carrie Moss, a certified application counselor, said the exchange Web site was not working when she tried to show patients their health care options.

And in Kentucky, by 9 a.m. the state-run insurance exchange was freezing up, with users unable to take even the first step of creating an account. Gwenda Bond, a spokeswoman, said that more than 1,000 applications had been filed online by 8:30 a.m., and that the “high volume of traffic” was to blame for the technical problems.

By late afternoon in the East, state exchanges reported much heavier traffic than expected to their Web sites. New York said that 10 million attempts had been made to reach its site, although with many people making multiple tries, it was not clear how many individuals that represented. And officials said the figure was so far beyond anything they had considered plausible that they were investigating the cause.

In Illinois, the exchange site had 70,000 visits from 65,000 people, with 1,100 applications submitted. Kentucky had nearly 78,000 unique visitors, who started nearly 4,700 applications and completed more than 2,900. Oregon’s exchange had 75,000 visitors, but with the state still working out some problems, it has postponed the ability to apply for coverage.

About 55,000 people went to Colorado’s exchange site, and 1,450 created accounts to allow them to start shopping for plans. Idaho’s exchange had more than 8,000 visitors, but it was not clear whether any of them were able to apply, because the applications were routed through the federal system.

In California, Peter Lee, the executive director of Covered California, said the state health exchange was running slowly because it had received five million hits on Tuesday, and he said workers were performing maintenance to speed it up. “Some of the folks have had a slow experience,” Mr. Lee said at Union Station in Los Angeles. “Surprise, surprise.”

Critics quickly seized on the technology problems as evidence that the health care law was deeply flawed.

Photo

Mauricio Correa talked to patients in East Los Angeles.Credit
Monica Almeida/The New York Times

“We should have never gotten to this point,” said Senator Orrin G. Hatch of Utah, the senior Republican on the Senate Finance Committee. “The Obama administration should have acknowledged the ample warning signs of problems in the exchanges and heeded the many calls for delay.”

“Tens of thousands of Americans die each year just because they don’t have health insurance,” the president said. “Millions more live with the fear that they’ll go broke if they get sick. And today, we begin to free millions of our fellow Americans from that fear.”

An error has occurred. Please try again later.

You are already subscribed to this email.

In all, the Obama administration expects about seven million people to be enrolled in private health plans through the exchanges in 2014. Millions more are expected to get coverage under an expansion of Medicaid, although many states are refusing to go along with the expansion because their leaders are opposed to the health care law.

Federal officials would not say how many people successfully enrolled in health plans on Tuesday through the federal site. They also conceded that factors beyond high use had contributed to the Internet delays.

Throughout the day, users had difficulty creating online accounts and therefore could not see their health plan options or premiums. Users were asked to provide answers to three “security questions,” but many could not choose or write in any security question.

In a telephone conference call on Tuesday afternoon, Marilyn B. Tavenner, the administrator of the federal Centers for Medicare and Medicaid Services, said the problem was “corrected earlier today” on the federal Web site. But within 10 minutes, journalists informed Ms. Tavenner that the problem persisted, and she expressed surprise.

“This is Day 1 of a six-month process,” Ms. Tavenner said. “We’re in Day 1 of a marathon, not a sprint, and we need your help.”

The Obama administration had stressed that consumers could also call customer service centers or seek out clinics, nonprofit organizations and other groups designated as “navigators” to help with the enrollment process. But those systems also stumbled, dependent on the same glitch-plagued computer systems.

In Louisville, Ky., dozens of people went to Jefferson Community and Technical College, where seven counselors were ready to help them sign up for coverage and find out if they were eligible for federal subsidies. But because of problems with the Kentucky exchange Web site, the counselors mostly had to abandon their laptops and hand out paper applications or ask people to come back another day.

When Christie Hartlage tried to start the online application process for Keairra Taylor, 23, a warning message popped up on her laptop screen: “An unknown error has occurred.”

Ms. Taylor, who works part time at a day-care center while she studies to become a social worker, said she had symptoms of multiple sclerosis and a growing pile of medical bills from frequent hospitalizations. Her 1-year-old son is covered by Medicaid, but Ms. Taylor said she currently did not qualify for the program, despite earning less than $7,200 a year.

If Ms. Taylor qualifies for Medicaid under an expansion of the program that is taking place in Kentucky under the health care law, “It will feel like a weight has been lifted,” she said.

In Columbus, Patrick Gamble, an employee of Columbus Neighborhood Health Centers who was helping people enroll, said he had similar frustrations with the federally run exchange on Tuesday.

“The Web site is log-jammed right now,” he said. “Everyone is eager for information.”

Asked what patients thought about the new exchange, Mr. Gamble added, “Nobody has gotten that far.”

Although some people who tried to explore their options or enroll on Tuesday were frustrated, others said they did not mind waiting a little longer for the coverage, which will not take effect until Jan. 1. In Miami, Bertha Stewart, who tried unsuccessfully to apply for coverage at the Jessie Trice center, in Liberty City, a poor neighborhood, just smiled when the counselor helping her could not get the federal Web site to work.

“I am patient,” Ms. Stewart said, who is 53 and suffers from asthma. “That’s how you get stuff done. You are patient.”

Correction: October 1, 2013

An earlier version of this article misstated the deadline for enrolling in an insurance plan. It is March 31, not Jan. 1. (Insurance bought through the exchanges by Dec. 15 would take effect on Jan. 1.) The article also misstated the day of an announcement on healthcare.gov, the federally run exchange. It was posted on Tuesday, not on Monday.

A version of this article appears in print on October 2, 2013, on Page A1 of the New York edition with the headline: Opening Rush to Insurance Markets Hits Snags. Order Reprints|Today's Paper|Subscribe